1. Field of the Invention
The present invention relates to a control apparatus for a traction motor of a vehicle.
2. Description of Related Art
It is known that some of control apparatuses to control an electric motor used as a traction power source for an electric vehicle, a hybrid vehicle, or an electric train are configured to detect the rotation angle of the motor and control power supply to the motor in accordance with the detection result through a drive device such as an inverter. For example, refer to Japanese Patent Application Laid-Open No. 10-215504 (Patent document 1).
Patent document 1 describes a control apparatus which uses a resolver as a rotation angle sensor to detect the rotation angle of the motor. The resolver includes a rotor which rotates together with the output shaft of the motor and a stator, and is configured to output a rotation detection signal having a value varying depending on variation of the reluctance between the rotor and the stator thereof in accordance with the rotor position (accordingly, in accordance with the rotation angle of the motor). In the following description, to distinguish the stator and rotor of the resolver from those of the motor, the stator of the resolver is referred to as the resolver stator, and the rotor of the resolver is referred to as the resolver rotor.
When the resolver is of the two-phase type to output two rotation detection signals which are 90 degree apart from each other in electrical angle, it has a structure in which a primary coil is provided in one of the resolver rotor and the resolver stator, and a secondary coil is provided in the other. When the primary coil is applied with an excitation signal of a constant frequency, amplitude-modulated versions of the excitation signal are outputted from the secondary coil as the two rotation detection signals which are 90 degree apart from each other in electrical angle.
The rotation detection signals outputted from the resolver are analog signals. Accordingly, it is common to use an RDC (Resolver Digital Converter) in order to convert each of the rotation detection signals outputted from the resolver into digital angle data representing the rotation angle of the motor. The control apparatus controls power supply to the motor in accordance with the digital angle data received from the RDC.
For the RDC to output the digital angle data with high precision, it is ideal that the two rotation detection signals outputted from the resolver have the same amplitude, no offset, and a specific phase difference therebetween (90 degrees in the above example).
However, actually, the two rotation detection signals outputted from the resolver may have an amplitude difference therebetween, an offset, and a phase error due to manufacturing tolerance of the resolver, that is, the resolver-to-resolver variation in the shape, characteristics of the coils, the gap between the resolver rotor and the resolver stator etc.
When the rotation detection signal includes an error due to the manufacturing tolerance, also the angle data outputted from the RDC includes an error. In this case, the angle data outputted from the RDC become non-linear with respect to time although when the motor is rotating at a constant speed.
As a result, since the control apparatus controls power supply to the motor in accordance with the rotation angle of the motor erroneously determined by the angle data including an error received from the RDC, the motor cannot be controlled in an appropriate manner.
For example, when the vehicle runs at a constant speed and the vehicle driver presses the accelerator pedal at a constant force, if the motor is controlled in accordance with the angle data including the above error and accordingly not showing the correct rotation angle of the motor, the vehicle is applied with a longitudinal acceleration because of variation of torque of the motor caused by the above error although the vehicle driver does not change the pressing force to the accelerator pedal. This gives feeling of discomfort to the vehicle driver and passengers.
Other than the resolver as described above, there are other types of the rotation angle sensor for detecting the rotation angle of a motor, including an incremental encoder configured to output a pulse signal for each predetermined rotation angle range and output a pulse signal each time the motor rotates one turn, an absolute encoder configured to output angle data representing the present rotation angle for each predetermined rotation angle range, and a sensor constituted of a combination of a vertical hall element and a magneto-resistive element and configured to output a signal depending on the present rotation angle as disclosed in Japanese Patent Application Laid-Open No. 2008-185406 (Patent document 2). However, the output signal of each of the above encoders and the sensor may also include an error due to manufacturing tolerance as with the case of the above described resolver.
Incidentally, there is known a method for detecting the rotation angle of a motor, the method having a function of compensating for the error included in the rotation detection signal. For example, refer to Japanese Patent Application Laid-Open No. 10-170531 (Patent document 3). The rotation angle detecting method according to Patent document 3 is configured to measure the time necessary for the resolver rotor to rotate one turn when the motor is rotating at a constant speed, and calculate, on the basis of the measured time, m (m being a positive integer larger than 1 determined depending on the resolution of the resolver) angle data items as data reference θn (n being 1 to m). The angle data φn outputted from the resolver is compared with the angle data reference θn, and the difference therebetween is stored in a memory as a correction value for the angle data reference θn (n being 1 to m). To control the motor, each time the angle data φn is inputted, it is corrected by the correction value for the angle data φn stored in the memory.
According to the method described in Patent document 3, it is possible to compensate for the error included in the rotation detection signal due to manufacturing tolerance of the resolver, however, it involves a problem that the hardware scale of the control apparatus has to be increased.
That is, the method described in Patent document 3 needs to store the correction value for each predetermined angle range depending on the resolution of the resolver, a memory of a large capacity has to be provided.
In recent years, to control a traction motor with a high degree of accuracy, it is required to detect the rotation position of the motor at a resolution as high as possible. However, to address this requirement by the method described in Patent document 3, it is necessary to further increase the capacity of the memory to store the correction values.